Buffett’s MidAmerican Energy Holding Co.’s purchase of Las Vegas-based NV Energy Inc., announced Wednesday, and a May 28 gas utility deal were probably spurred by a month-long drop in stock prices that has made utilities more attractive to cash buyers, David Herr, a Philadelphia-based managing director with investment bank Duff & Phelps, said in a phone interview.

“The sell-off probably provided a good opportunity to get these deals moved across the line,” said Herr, who leads Duff & Phelps’s energy and mining practice. Buyers looking to strike an all-stock transaction that’s typical for large-scale mergers won’t feel the same pressure to act because “both sides of the equation have likely gone down with the market,” he said.

After climbing to a five-year high April 30, the Standard and Poor’s 500 Utilities Index has dropped 8.5 percent on investor concerns that rising interest rates will make utility dividends less attractive.

The sector may see additional cash purchases of utilities for about $1 billion to $5 billion if interest rates don’t rise, Herr said. Mega-mergers of $20-billion power companies “would be tough in the current pricing environment” because valuations remain relatively high and regulators may seek to funnel merger savings to consumers through costly concessions.